Learning how high your grow lights should be above your plants is one of many things to know about growing cannabis. If you’ve got your grow tent, your soil, and your seedlings. You should know that as an indoor cannabis grower, few things will make a bigger difference in your grow kit than the setup of your grow lights.
Today’s growers are largely opting for LED lighting — this type of grow light emits less heat, which means less wasted energy and different distances from the plants. Below, we’ll look at how light height affects different stages of cannabis growth.
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How to Place Grow Lights
The performance of a 600W LED grow light will vary significantly with distance from the plant’s canopy. For this reason, you should mount your lights at different distances for different stages of plant growth.
The seedling stage typically lasts between 2 and 3 weeks.
When you plant your cannabis seedlings, place your light (600W LED for this example) between 2 and 3 feet above the plant’s canopy. This is the maximum distance from the plants, and maintaining this distance is important to prevent drying out the seedlings and soil. Remember that seedlings are very delicate and need gentle light.
When the seedlings sprout and establish roots, you can start inching the lights closer — but avoid going closer than 2 feet when it’s still in the seedling stage.
Place LED grow lights between 1 and 2 feet above the top of the canopy once your cannabis plants have entered the vegetative stage. They can be closer because the plants are now in a more intensive photosynthesis phase.
Yet more light is needed as cannabis plants reach the flowering stage. However, don’t move the grow lights closer — make sure they’re between 1½ and 2 feet above the top leaves.
At this stage, the plants may grow taller as they begin to bear fruit. Keeping your light at the same height keeps your cannabis plants from growing too tall. Note that if your lights are too close, it can make the plant grow too wide and cause damage.
What About Cannabis Clones?
If you’re cloning cannabis — taking cuttings from a mature plant and using them to grow an identical plant elsewhere — you’ll need to position your lights slightly differently.
Because cloning doesn’t start with seedlings, you will need to start with intense light. Place your LED grow light between 14 and 36 inches above the plant’s canopy, depending on how mature your clippings are.
How to Spot Excessive Light
How do you know when your grow lights are too close or too intense?
The key is to be able to spot the signs of damage before they become problems. Cannabis plants show distress when they’re receiving too much light, which most commonly manifests as something called “light burn.”
Light burn can be spotted by upward-pointing leaves, as well as by an effect known as bleaching. Bleaching can be seen in the yellow/white discoloration of those leaves that are nearest to the light source. You may also notice light burn in the yellowing of leaves, even as the veins stay green.
Irregular or stunted growth is another side effect of too much light. If you give a plant too much, it can expand faster than its own capacity allows, causing damage in the long run.
What About Non-LED Lighting?
You may still have some non-LED lights you’re trying to use until they expire. The most common types are High-Pressure Sodium (HPS), High-Intensity Discharge (HID), metal halide, and fluorescent lights.
Note that while it’s up to you whether you want to use these or simply get some new lights, LED lights are dramatically more efficient and low cost. They’re also better for the environment because they consume far less electricity — something we should aspire to when growing cannabis.
In general, non-LED lighting should be farther from your cannabis plant than LEDs because they are a.) hotter and b.) only come in certain wattage outputs.
For HID and metal halide lamps, start with around 19 to 26 inches of height and work your way up from there. Fluorescent lights may be able to stay closer to the cannabis leaves since they’re low-heat. Start around 6-12 inches for young crops and raise it to 12-16 inches once they’ve reached the vegetative/flowering states.
Act Like a Scientist
If you want scientific results, then act like a scientist! Take notes of what works and what doesn’t — that way, you’ll get a better handle on how your particular lamps behave and how different strains accept them. Happy growing!